Here, we describe the first documented case of BKPyV-associated PML in a patient with erythrodermic mycosis fungoides treated with Mogamulizumab, a CCR4-targeting monoclonal antibody that profoundly alters immune surveillance.<h4>Results</h4>We describe a patient with erythrodermic mycosis fungoides and long-standing immunological frailty, who developed neurological symptoms during Mogamulizumab therapy. The gene discussed is CCR4; the disease is mycosis fungoides.